Bootstrapped switches are widely used in various types of circuit applications. In one example, a bootstrapped drive circuit may be used to provide an output voltage generated from a charged capacitor (which will be referred to as a “boot” capacitor—CBOOT) to drive the gate of a MOSFET switch and turn on the MOSFET switch, which will be referred to as a bootstrapped switch. The bootstrapped drive circuit is clocked by a clock signal that switches the bootstrapped switch between an off state in which the output voltage is low and the boot capacitor is charged, and an on state in which the output voltage is high to turn on the bootstrapped MOSFET switch. This technique may be used, for example, to connect an input signal received on the source terminal of the MOSFET switch to an output load connected to the drain terminal of the MOSFET switch.
A bootstrapped drive circuit may be typically configured to provide the output voltage when in the on state at a high enough voltage level to ensure that the MOSFET switch remains on and with a voltage offset based on the input signal. The output voltage is provided at a voltage that is equal to the voltage stored across the boot capacitor plus the voltage level of the input signal. In the example above the voltage seen at the gate of the MOSFET would then be equal to the voltage stored across the capacitor plus the voltage level of the input signal and would track the input signal when the bootstrapped switch was on. This maintains a relatively constant gate to source voltage for the MOSFET switch across all input signal levels, which provides significant linearity improvement when using such a bootstrapped switch in, for example, a sampling circuit like a voltage sample and hold, compared to using a MOSFET switch with its gate voltage driven high to a fixed voltage. These aspects of a bootstrapped switching circuit, which includes the bootstrapped drive circuit and the bootstrapped switch itself, provide advantages that may be utilized in many other switching circuit applications. Because bootstrapped switches are widely used in the above described example application and in many other varied switching circuit applications, it would provide an advantage, therefore, to have an improved bootstrapped switching circuit having improved performance with faster turn-on time.
Overview
In one example embodiment the method, apparatus and system comprises a switching circuit that has an input for receiving an input signal and includes a bootstrapped drive circuit having an input switch coupled to the input and including a first output. The bootstrapped drive circuit is configured to receive the input signal at the input switch and provide a first drive signal on the first output responsive to the input switch being switched on by a second drive signal. The switching circuit includes a switching loop coupled to the input of the switching circuit. The switching loop has a second output coupled to the input switch of the bootstrapped drive circuit. The switching loop is configured to provide the second drive signal on the second output to the input switch in the bootstrapped drive circuit to turn on the input switch. The embodiment may further comprise a bootstrapped switch coupled to the first output, where the switch is configured to turn on in response to the first drive signal generated by the bootstrapped drive circuit. The switch may receive the input signal at an input and pass it to an output when turned on.
In another embodiment a switching circuit has an input for receiving an input signal and includes a bootstrapped drive circuit having an input switch coupled to the input and including a first output. The bootstrapped drive circuit is configured to receive the input signal at a first input switch and provide a first drive signal on the first output in response to the first input switch being switched on by a second drive signal. The switching circuit includes a switching loop coupled to the input of the switching circuit. The switching loop has a second output coupled to the first input switch in the bootstrapped drive circuit. In this embodiment, the switching loop may further comprise a second input switch coupled to the input of the switching circuit, and the switching loop may be configured to provide the second drive signal to the first input switch in the bootstrapped drive circuit responsive to the second input switch in the switching loop being switched on by the second drive signal. In an alternative embodiment, the first input switch of the bootstrapped drive circuit may be coupled to the first output through a first boot capacitor and the second input switch in the switching loop may be coupled to the second output through a second boot capacitor, providing a boot capacitor for each of the bootstrapped drive circuit and the switching loop. In another alternative, the bootstrapped drive circuit may further comprise a first output switch, wherein the first boot capacitor is coupled to the first output through the first output switch and the switching loop may further comprise a second output switch, wherein the second boot capacitor is coupled to the second output through the second output switch. The first and second output switches may switch on in response to at least one clock signal, for example, a clock signal input to both switches, and provide the first and second drive signals.
In another embodiment a switching circuit has an input for receiving an input signal and includes a bootstrapped drive circuit having an input switch coupled to the input and including a first output. The bootstrapped drive circuit is configured to receive the input signal at the input switch and provide a first drive signal on the first output responsive to the input switch being switched on by a second drive signal. The switching circuit includes a switching loop coupled to the input of the switching circuit. The switching loop has a second output coupled to the input switch of the bootstrapped drive circuit. In this embodiment, the input switch of the bootstrapped circuit may comprise a shared input switch that functions as an input switch of both the bootstrapped drive circuit and the switching loop. The shared input switch may be coupled to the first output and second output through a boot capacitor and the boot capacitor may function as a boot capacitor for both the bootstrapped drive circuit and the switching loop. In an alternative of this embodiment the bootstrapped drive circuit further comprises a first output switch and the boot capacitor is coupled to the first output through the first output switch. The switching loop may comprise a second output switch and the boot capacitor may be coupled to the second output through the second output switch. The first and second output switches may switch on in response to at least one clock signal, for example, a clock signal input to both switches, and provide the first and second drive signals.
In still another embodiment the switching circuit comprises a buffer coupled between the input of the switching circuit and the input of the switching loop. In an alternative of this embodiment, the buffer may be coupled between the input of the switching circuit and both of the input of the bootstrapped drive circuit and the input of switching loop.
In a further embodiment a pass gate may be coupled between the input of the switching loop and the second output, and turn on in response to at least one clock signal. In this embodiment the switching loop provides the second drive signal to the input switch of the bootstrapped drive circuit when the pass gate is turned on. In alternatives of this embodiment, the pass gate may be implemented in place of the input switch of the switching loop or implemented in parallel with the input switch of the switching loop.